A circuit protection CT disconnection discrimination method and computer equipment
By adding a zero-sequence voltage condition on the opposite side to the CT disconnection discrimination logic, combined with dual-end discrimination, the problem of misjudgment of high-resistance faults was solved, and accurate identification of CT disconnection and normal operation of protection devices were achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- XJ ELECTRIC CO LTD
- Filing Date
- 2024-12-19
- Publication Date
- 2026-06-23
Smart Images
Figure CN122267684A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of power protection technology, specifically relating to a method for detecting line breakage in a line protection CT and a computer device. Background Technology
[0002] Currently, line protection devices mainly use two methods to detect CT (current transformer) disconnection: single-end detection and double-end detection. Single-end detection is based on electrical quantities on one side and can only reflect the operating status at the protection installation location, which has significant limitations when applied to high-voltage line protection. Thanks to the widespread use of fiber optic channels in line protection, double-end detection can analyze and compare the electrical quantities at both ends of the high-voltage line during operation and make a judgment, resulting in a more accurate and effective judgment compared to single-end detection.
[0003] Commonly used two-terminal discrimination logic in existing technologies includes: no current in the faulty phase, zero-sequence current generated by the sensing side, and no zero-sequence current generated by the sensing side. However, for special fault conditions occurring in high-voltage transmission lines, such as high-resistance faults, these commonly used two-terminal discrimination logics have limitations and are prone to misjudgment. This is because high-resistance faults are characterized by extremely high transition resistance, reaching tens of kiloohms, extremely small current in the faulty phase (almost no current), zero-sequence current in the transmission line reaching the CT disconnection threshold, and extremely low zero-sequence voltage that can maintain the threshold requirement for a long time. In other words, the fault characteristics of high-resistance faults are highly consistent with these commonly used two-terminal discrimination logics in terms of certain electrical quantities. Once a high-resistance fault is misjudged as a CT disconnection, the protection is blocked, causing the protection device to fail to operate. Prolonged operation under fault conditions can easily expand the fault range and lead to more serious consequences. Summary of the Invention
[0004] The purpose of this invention is to provide a method and computer device for detecting CT disconnection in line protection, in order to solve the problem that the CT disconnection detection logic commonly used in the prior art is used to misjudge high resistance faults as CT disconnection, causing the protection device to fail to operate and thus expanding the fault range.
[0005] To solve the above-mentioned technical problems, the present invention provides a method for determining the open circuit of a line protection current transformer (CT), comprising the following steps: 1) Obtain the electrical parameters at both ends of the line; 2) If the obtained electrical quantity parameters meet all CT disconnection judgment conditions and continue for a set time, then the CT on this side of the line is determined to be disconnected; where, the electrical quantity parameters meeting all CT disconnection judgment conditions means that the electrical quantity parameters meet all preliminary CT disconnection judgment conditions and the zero-sequence current on the opposite side in the electrical quantity parameters is less than the zero-sequence current threshold value on the opposite side.
[0006] Furthermore, the preliminary criteria for determining CT disconnection specifically include: the local zero-sequence current in the electrical quantity parameters is greater than the local zero-sequence current threshold value; the local zero-sequence voltage in the electrical quantity parameters is less than the local zero-sequence voltage threshold value; and the opposite zero-sequence voltage in the electrical quantity parameters is less than the opposite zero-sequence voltage no-voltage threshold value.
[0007] Furthermore, all preliminary criteria for CT line disconnection include: the abnormal phase difference current on the local side and the opposite side of the line in the electrical quantity parameters is greater than the phase difference current threshold value.
[0008] Furthermore, all preliminary criteria for CT line disconnection include: the zero-sequence differential current on the local and opposite sides of the line in the electrical quantity parameters is greater than the zero-sequence differential current threshold value.
[0009] Furthermore, the preliminary criteria for determining CT disconnection also include: the current in a certain phase on this side of the electrical quantity parameter is less than the current-free threshold value of the phase on this side.
[0010] Furthermore, the zero-sequence voltage threshold value on the opposite side is the zero-sequence voltage of the protection device for K cycles before protection activation.
[0011] Furthermore, the local zero-sequence current threshold value is the smaller of 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
[0012] Furthermore, the phase difference current threshold value is the smaller of 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
[0013] Furthermore, the zero-sequence differential current threshold value is the smaller of 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
[0014] To address the aforementioned technical problems, the present invention also provides a computer device, including a processor, which executes a computer program to implement the steps of the above-described method for detecting line protection CT disconnection.
[0015] This invention is an improved invention with the following advantages: Addressing the situation where the fault characteristics of high-resistance faults and the electrical quantity parameters for CT open-circuit identification highly match, this invention adds an electrical quantity capable of identifying whether a CT open-circuit fault is due to a high-resistance fault: the zero-sequence current at the opposite end. In the event of a high-resistance fault, the zero-sequence current on the opposite side exists and is greater than or equal to the zero-sequence current threshold on the opposite side. However, in the event of a CT open-circuit fault, the zero-sequence current on the opposite side is less than the zero-sequence current threshold on the opposite side. Therefore, a new condition is added to the initial CT open-circuit identification criteria: the zero-sequence voltage on the opposite side is less than the zero-sequence voltage threshold on the opposite side. Only when both the initial and added conditions are met is a CT open-circuit fault identified. This ensures normal identification when a current transformer (CT) open-circuit occurs, blocking related protections and preventing maloperation of the relay protection device. Furthermore, combined with this added condition, it prevents misidentification of a high-resistance ground fault under a certain load as a CT open-circuit fault, ensuring the relay protection device operates normally and effectively to clear the fault, avoiding the phenomenon of protection device failure to operate and thus expanding the fault range. Attached Figure Description
[0016] Figure 1 This is a flowchart of the line protection CT disconnection detection method of the present invention; Figure 2 This is a schematic diagram of the line protection CT disconnection logic of the present invention. Detailed Implementation
[0017] The main concept of this invention is to add a new condition to the preliminary judgment condition of CT disconnection: the zero-sequence voltage on the opposite side is less than the zero-sequence voltage threshold value on the opposite side. A CT disconnection is only identified when both the preliminary judgment condition and the newly added condition are met. This new condition prevents misjudgment of a high-resistance ground fault under a certain load as a CT disconnection, ensuring the relay protection device operates normally and effectively clears the fault in a timely manner. Based on this main concept, a line protection CT disconnection judgment method and a computer device of this invention can be realized. To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.
[0018] An embodiment of a method for detecting line protection CT disconnection: The present invention provides a method for detecting line protection CT disconnection, the basic process of which is as follows: Figure 1 As shown, the specific process is as follows: Step 1: Collect voltage and current data on the line side (M side) and the opposite side (N side).
[0019] Step two: Calculate electrical quantity parameters based on the data collected in step one, specifically including the local zero-sequence current 3. I 0 (3)I 0 represents the zero-sequence current on this side), and 3 represents the zero-sequence voltage on this side. U 0 and the zero-sequence current on the opposite side 3 I 0, Zero-sequence voltage on the opposite side 3 U 0, the electrical parameters of the opposite side are transmitted to this side via optical transmission. Then, the phase differential current and zero-sequence differential current on the line are calculated.
[0020] Step 3: Determine whether all the electrical parameters at both ends of the line meet the CT disconnection criteria proposed in this invention. If any one condition is not met, return to Step 1 to re-acquire voltage and current data. If all the CT disconnection criteria are met, proceed to Step 4.
[0021] This invention relies on the fiber optic longitudinal transmission mode for high-voltage line protection and employs a dual-end criterion. All CT disconnection detection conditions include preliminary CT disconnection detection conditions and additional conditions added in this invention to distinguish between CT disconnection and high-resistance faults. The overall logic of these conditions is as follows: Figure 2 As shown.
[0022] The preliminary criteria for CT line breakage identification are those commonly used in CT line breakage identification but cannot accurately identify high-resistance faults. These criteria include: Condition 1: The zero-sequence current on this side is greater than the zero-sequence current threshold value on this side. When a CT disconnection occurs, due to a current transformer fault, the primary current of a certain phase cannot be collected. At this time, the current of that phase collected by the protection device is zero, resulting in a three-phase current imbalance. The protection device's sensing system generates a zero-sequence current. Therefore, determining whether a zero-sequence current is generated on this side can identify a CT disconnection. That is, the zero-sequence current on this side is 3... I If 0 is greater than the zero-sequence current threshold value on this side, it indicates that there is a sudden change in the current of one phase on this side, and the current on this side is abnormal.
[0023] Condition 2: The local zero-sequence voltage is less than the set local zero-sequence voltage threshold. A significant characteristic of a CT disconnection is that it only affects current acquisition and does not cause a significant change in voltage, unlike faults on high-voltage transmission lines, which are often accompanied by simultaneous changes in both current and voltage. As a typical distinction between a CT disconnection and a high-voltage transmission line fault, determining whether the local zero-sequence voltage is less than the local zero-sequence voltage threshold is one way to differentiate between high-resistance faults and CT disconnections.
[0024] Condition 3: The zero-sequence voltage on the opposite side is less than the zero-sequence voltage threshold value on the opposite side. Similar to condition 2), when only a CT disconnection occurs, the voltage of the high-voltage transmission line system will not fluctuate significantly. By adding a judgment on whether the zero-sequence voltage on the opposite side is in a normal state, i.e., whether its zero-sequence voltage is less than the zero-sequence voltage no-voltage threshold value on the opposite side, this is a way to judge CT disconnection. Furthermore, considering the floating threshold of zero-sequence voltage, the zero-sequence voltage no-voltage threshold value on the opposite side is set as the zero-sequence voltage of K cycles (K can be 5) before the protection starts. Generally, it is less than 0.5V. This can ensure that under the lower limit of the precise working voltage (0.25V-80V), the protection device on the opposite side does not operate and does not start, allowing CT disconnection judgment.
[0025] Furthermore, based on the above, differential current can also be taken into account when determining CT line breakage, resulting in the following conditions 4 and 5.
[0026] Condition 4: The phase difference current between the abnormal phases on this side and the opposite side of the line is greater than the phase difference current threshold value. A prominent symptom of a CT disconnection is that under normal operating conditions, there is no current in a certain phase. If a CT disconnection occurs, the protection device on this side will collect a current of no current, while the protection device on the opposite side will collect a current of normal. At this time, the abnormal phase difference current at both ends (i.e., this side and the opposite side) is calculated to be greater than the corresponding phase difference current threshold value, and the result is determined to be an abnormal current situation at one end.
[0027] Condition 5: The zero-sequence differential current on both sides of the line is greater than the zero-sequence differential current threshold. When a current transformer (CT) breaks on either side of a high-voltage transmission line, the current in the broken phase decreases to a zero-current state. At this time, the other phases operate normally, and the three-phase imbalance generates a zero-sequence current. No abnormality occurs on the opposite side, and no zero-sequence current is generated. The fiber optic channel calculates the zero-sequence differential current generated at both ends of the system by comparing the electrical quantities at both ends. Therefore, a zero-sequence differential current greater than the zero-sequence differential current threshold can be used to determine whether a CT break has occurred.
[0028] In addition, based on the direct manifestation of CT line breaks, condition 6 can also be set for preliminary CT line break identification.
[0029] Condition 6: The current in a certain phase on this side is less than the current-free threshold value for that phase. When a CT disconnection occurs, the most direct manifestation is that the current in a certain phase suddenly drops to a state of no current. Analyzing and judging whether the currents of the three phases A / B / C are lower than the current-free threshold value is the simplest and most direct way to determine if a CT disconnection has occurred.
[0030] The specific condition that can be used to distinguish between a CT open circuit and a high-resistance fault is condition 7.
[0031] Condition 7: The zero-sequence current on the opposite side is less than the zero-sequence current threshold value on the opposite side. For high-resistance faults on high-voltage transmission lines, the fault impedance is very large, reaching over 300Ω, and even several kΩ under extreme conditions. At this time, the fault phase current decreases to below zero current (0.04 times In, where In is the rated current of 1A or 5A), satisfying the fault phase zero-sequence current condition. The fault causes a zero-sequence current to be generated on the line, satisfying the zero-sequence differential current, phase-specific differential current, and local zero-sequence current exceeding the discrimination threshold value. The phase voltage drop is not significant, and the zero-sequence voltage on the opposite side is 3... U The voltage is relatively small, and even after exceeding the CT disconnection discrimination time, it is still no greater than 1.5V. However, the zero-sequence current on the opposite side exists and is greater than or equal to the zero-current threshold. That is, when it is determined that the zero-sequence current on the opposite side is less than the zero-sequence current threshold value on the opposite side, the protection device will not misjudge the high-resistance fault occurring under this load condition as a CT disconnection. The protection device will not block the zero-sequence protection, the differential action setting will not be raised to the differential action setting value after the CT disconnection, and the zero-sequence protection direction will not be withdrawn. When the action conditions are met, the protection device can still operate normally, thereby effectively isolating the fault and reducing the impact.
[0032] Among them, the local zero-sequence current threshold value in condition 1, the zero-sequence differential current threshold value in condition 5, and the phase differential current threshold value in condition 4 can be set to the same value, which is the smaller of the three values: 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
[0033] Step four: If all the discrimination conditions in step three are met and the set time is maintained, the device determines that the CT on this side is disconnected rather than a high-resistance fault, and reports "*phase CT disconnected", where "*" is A / B / C; otherwise, it does not determine that the CT is disconnected, the protection is normally open, and the relevant protection is not locked.
[0034] An embodiment of a computer device: An embodiment of a computer device according to the present invention includes a memory, a processor, an internal bus, and a computer program stored in the memory. The processor and the memory communicate and interact with each other via the internal bus. The processor executes the computer program to implement the steps of the method described in an embodiment of the line protection CT disconnection detection method of the present invention. The processor can be a microprocessor (MCU), a programmable logic device (FPGA), or other processing devices; the memory can be various types of memory that store information using electrical energy, such as RAM or ROM, or other types of memory.
[0035] In summary, this invention can correctly identify and block relevant protections when a current transformer (CT) disconnects, ensuring that the relay protection device does not malfunction. Furthermore, when a high-resistance grounding fault occurs under a certain load, it can avoid misjudging it as a CT disconnection, thus protecting the relay protection device and ensuring timely and effective fault isolation.
[0036] Specific implementation methods have been given above, but the present invention is not limited to the described implementation methods. The basic idea of the present invention lies in the above basic scheme. For those skilled in the art, designing various modified models, formulas, and parameters based on the teachings of the present invention does not require creative effort. Changes, modifications, substitutions, and variations made to the implementation methods without departing from the principles and spirit of the present invention still fall within the protection scope of the present invention.
Claims
1. A method for detecting line protection CT disconnection, characterized in that, Includes the following steps: 1) Obtain the electrical parameters at both ends of the line; 2) If the obtained electrical quantity parameters meet all CT disconnection judgment conditions and continue for a set time, then the CT on this side of the line is determined to be disconnected; where, the electrical quantity parameters meeting all CT disconnection judgment conditions means that the electrical quantity parameters meet all preliminary CT disconnection judgment conditions and the zero-sequence current on the opposite side in the electrical quantity parameters is less than the zero-sequence current threshold value on the opposite side.
2. The line protection CT disconnection detection method according to claim 1, characterized in that, The specific criteria for preliminary diagnosis of CT line breaks include: The local zero-sequence current in the electrical quantity parameters is greater than the local zero-sequence current threshold value; The local zero-sequence voltage in the electrical quantity parameters is less than the local zero-sequence voltage threshold value; The zero-sequence voltage on the opposite side in the electrical quantity parameters is less than the zero-sequence voltage threshold value on the opposite side.
3. The method for determining line protection CT disconnection according to claim 2, characterized in that, All preliminary criteria for CT line disconnection also include: the abnormal phase difference current on the local side and the opposite side of the line in the electrical quantity parameters is greater than the phase difference current threshold value.
4. The method for determining line protection CT disconnection according to claim 2, characterized in that, All preliminary criteria for CT line disconnection also include: the zero-sequence differential current on the local and opposite sides of the line in the electrical quantity parameters is greater than the zero-sequence differential current threshold value.
5. The method for determining line protection CT disconnection according to claim 2, characterized in that, All preliminary criteria for CT line disconnection also include: the current of a certain phase on this side in the electrical quantity parameters is less than the no-current threshold value of the phase on this side.
6. The line protection CT disconnection detection method according to claim 2, characterized in that, The zero-sequence voltage threshold value on the opposite side is the zero-sequence voltage of the protection device for K cycles before protection is activated.
7. The method for determining line protection CT disconnection according to claim 2, characterized in that, The zero-sequence current threshold value on this side is the smaller of 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
8. The method for determining line protection CT disconnection according to claim 3, characterized in that, The phase difference current threshold value is the smaller of 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
9. The method for determining line protection CT disconnection according to claim 4, characterized in that, The zero-sequence differential current threshold value is the smaller of 0.8 times the differential operating current setting, 0.1 times the rated current, and the zero-sequence starting current setting.
10. A computer device, comprising a processor, characterized in that, The processor is used to execute a computer program to implement the steps of the method according to any one of claims 1 to 9.